I use the Park tool chain checker and I wait until the chain is nearly 1.00 and then I change the chain and cassette. I find when the chain is between 0.75 and 1.00 bicycles still perform well without any problems shifting and I do not get any skipping when pushing hard pedaling. These type of issues may begin to occur when the chain is > 1.00 so at that point I change it and the cassette.